Grant J. Mathews

r-process nucleosynthesis in the high-entropy supernova bubble

We show that the high-temperature, high-entropy evacuated region outside the recent neutron star in a core-collapse supernova may be an ideal r-process site. In this high-entropy environment it is possible that most nucleons are in the form of free neutrons or bound into alpha particles. Thus, there can be many neutrons per seed nucleus even though the material is not particularly neutron rich. The predicted amount of r-process material ejected per event from this environment agrees well with that required by simple galactic evolution arguments. When averaged over regions of different neutron excess in the supernova ejecta, the calculated r-process abundance curve can give a good representation of the solar-system r-process abundances as long as the entropy per baryon is sufficiently high. Neutrino irradiation may aid in smoothing the final abundance distribution. 60 refs.

Neutron Capture Elements in s -Process-rich, Very Metal-poor Stars

We report abundance estimates for neutron capture elements, including lead (Pb), and nucleosynthesis models for their origin, in two carbon-rich, very metal-poor stars, LP 625-44 and LP 706-7. These stars are subgiants whose surface abundances are likely to have been strongly aUected by mass transfer from companion asymptotic giant branch (AGB) stars that have since evolved to white dwarfs. The detections of Pb, which forms the —nal abundance peak of the s-process, enable a comparison of the abundance patterns from Sr (Z \ 38) to Pb (Z \ 82) with predictions of AGB models. The derived chemical compositions provide strong constraints on the AGB stellar models, as well as on s-process nucleosynthesis at low metallicity. The present paper reports details of the abundance analysis for 16 neutron capture elements in LP 625-44, including the eUects of hyper—ne splitting and isotope shifts of spectral lines for some elements. A Pb abundance is also derived for LP 706-7 by a reanalysis of a previously observed spectrum. We investigate the characteristics of the nucleosynthesis pathway that produces the abundance ratios of these objects using a parametric model of the s-process without adopting any speci—c stellar model. The neutron exposure q is estimated to be about 0.7 mbarn~1, signi—cantly larger than that which best —ts solar system material, but consistent with the values predicted by models of moderately metal-poor AGB stars. This value is strictly limited by the Pb abundance, in addition to those of Sr and Ba. We also —nd that the observed abundance pattern can be explained by a few recurrent neutron exposures and that the overlap of the material that is processed in two subsequent exposures is small (the overlap factor r D 0.1).

Evolution of heavy-element abundances as a constraint on sites for neutron-capture nucleosynthesis

Observed heavy-element abundance ratios in the halo and the disk are compared with one-zone models of galactic chemical evolution. These comparisons provide useful insight into the kinds of stellar environments responsible for r- and s-process nucleosynthesis. The growth of r-process material appears to be associated with the ejection rate of material from massive stars. Low-mass Type II supernovae are slightly favored if the r-process is primary. The growth of s-process material is consistent with production in intermediate-mass stars

New Nuclear Reaction Flow during r-Process Nucleosynthesis in Supernovae: Critical Role of Light, Neutron-rich Nuclei

We study the role of light, neutron-rich nuclei during r-process nucleosynthesis in supernovae. Most previous studies of the r-process have concentrated on the reaction flow of heavy, unstable nuclei. Although the nuclear reaction network includes a few thousand heavy nuclei, only limited reaction flow through light nuclei near the stability line has been used in those studies. However, in a viable scenario of the r-process in neutrino-driven winds, the initial condition is a high-entropy hot plasma consisting of neutrons, protons, and electron-positron pairs experiencing an intense flux of neutrinos. In such environments, light nuclei, as well as heavy nuclei, are expected to play important roles in the production of seed nuclei and r-process elements. Thus, we have extended our fully implicit nuclear reaction network so that it includes all nuclei up to the neutron-drip line for Z ≤ 10, in addition to a larger network for Z ≥ 10. In the present nucleosynthesis study, we utilize a wind model of massive Type II supernova explosions to study the effects of this extended network. We find that a new nuclear reaction flow path opens in the very light, neutron-rich region. This new nuclear reaction flow can change the final heavy-element abundances by as much as an order of magnitude.

Probing the early universe: A Review of primordial nucleosynthesis beyond the standard Big Bang

Abstract The scientific literature is rich with studies of non-standard primodial nucleosynthesis. We review a number of these variants on the simplest standard big bang (SBB) nucleosynthesis model and discuss some of the impact these studies have had on cosmology and particle physics. Primordial nucleosynthesis is one of the earliest probes of the universe, and non-standard nucleosynthesis models allow for an exploration of the range of conditions which might have prevailed during the first few minutes. In particular, nonstandard models often allow for a larger range of conditions to be present in the early universe than those allowed by the SBB while satisfying observational constraints such as the inferred primordial isotopic abundances and the number of neutrino species derived from recent e+e− collider experiments. By considering alternatives to the SBB we can also determine the model dependence of the constraints imposed on particle physics and cosmology from primordial nucleosynthesis.

Observational Constraints on Dark Radiation in Brane Cosmology

Center for Astrophysics, Department of Physics,University of Notre Dame, Notre Dame, IN 46556(Dated: February 5, 2008)We analyze the observational constraints on brane-world cosmology whereby the universe is de-scribed as a three-brane embedded in a five-dimensional anti-de Sitter space. In this brane-universecosmology, the Friedmann equation is modified by the appearance of extra terms which derive fromexistence of the extra dimensions. In the present work we concentrate on the “dark radiation”term which diminishes with cosmic scale factor as a

AMPLIFICATION OF INTERSTELLAR MAGNETIC FIELDS BY SUPERNOVA-DRIVEN TURBULENCE

Several lines of evidence suggest that magnetic fields grow rapidly in protogalactic and galactic environments. However, mean field dynamo theory has always suggested that the magnetic fields grow rather slowly, taking of order a Hubble time to reach the observed values. The theoretical difficulties only become worse when the system has a high magnetic Reynolds number, as is the case for galactic and protogalactic environments. The discrepancy can be reconciled if fast processes for amplifying the magnetic field could operate. Following the 2001 work of Balsara and coworkers, we show that an interstellar medium that is dominated by realistic energy input from supernova explosions will naturally become a strongly turbulent medium with large positive and negative values of the kinetic helicity. Even though the medium is driven by compressible motions, the kinetic energy in this high Mach number flow is mainly concentrated in solenoidal rather than compressible motions. These results stem from the interaction of strong shocks with each other and with the interstellar turbulence they self-consistently generate in our model. Moreover, this interaction also generates large kinetic helicities of either sign. The turbulent flow that we model has two other characteristics of a fast dynamo: magnetic energy growth independent of scale and a growth time that is comparable to the eddy turnover time. This linear phase of growth permits the field to grow rapidly until the magnetic energy reaches about 1% of the kinetic energy. At that stage, other astrophysical processes for producing magnetic fields can take over. Energetics, power spectra, statistics, and structures of the turbulent flow are studied here. Shock-turbulence interaction is shown to be a very general mechanism for helicity generation and magnetic field amplification, with applicability to damped Ly? systems, protogalaxies, the Galaxy, starburst galaxies, the intracluster medium, and molecular clouds.Several lines of evidence suggest that protogalactic and galactic environments manage to grow magnetic field very rapidly. This makes the theoretical problem of achieving rapid growth of magnetic fields in such environments difficult. Following Balsara, Benjamin and Cox (2001), we show that a multiphase ISM that is dominated by realistic energy inputs via supernova explosions will naturally become a strongly turbulent medium with large positive and negative values of the fluid helicity. The resultant interstellar turbulence seems to have some of the essential characteristics of a fast dynamo. Magnetic field growth in such a turbulent dynamo is studied in this paper. It is shown that the field undergoes a linear phase of evolution where it has a time of growth that is comparable to the eddy turn-over time. Energetics, power spectra, statistics and structures of the turbulent dynamo are studied here. It is shown that despite the fact that the ISM is driven by compressive motions, the kinetic energy in this high Mach number flow is mainly concentrated in solenoidal motions rather than compressive motions.

Cold Ideal Equation of State for Strongly Magnetized Neutron Star Matter: Effects on Muon Production and Pion Condensation

Neutron stars with very strong surface magnetic fields have been suggested as the site for the origin of observed soft gamma repeaters (SGRs). In this paper we investigate the influence of such strong magnetic fields on the properties and internal structure of these strongly magnetized neutron stars (magnetars). We study properties of a degenerate equilibrium ideal neutron-proton-electron (npe) gas with and without the effects of the anomalous nucleon magnetic moment in a strong magnetic field. The presence of a sufficiently strong magnetic field changes the ratio of protons to neutrons as well as the neutron drip density. We also study the appearance of muons as well as pion condensation in strong magnetic fields. We discuss the possibility that boson condensation in the interior of magnetars might be a source of SGRs.

Big Bang nucleosynthesis with a new neutron lifetime

We show that the predicted primordial helium production is significantly reduced when new measurements of the neutron lifetime and the implied enhancement in the weak reaction rates are included in big-bang nucleosynthesis. Therefore, even if a narrow uncertainty in the observed helium abundance is adopted, this brings the constraint on the baryon-to-photon ratio from BBN and the observed helium into better accord with the independent determination of the baryon content deduced from the WMAP spectrum of power fluctuations in the cosmic microwave background and measurements of primordial deuterium in narrowline quasar absorption systems at high redshift.

Nuclear shell model calculations of neutralino-nucleus cross sections for 29Si and 73Ge.

We present the results of detailed nuclear shell model calculations of the spin-dependent elastic cross section for neutralinos scattering from \si29 and \ge73. The calculations were performed in large model spaces which adequately describe the configuration mixing in these two nuclei. As tests of the computed nuclear wave functions, we have calculated several nuclear observables and compared them with the measured values and found good agreement. In the limit of zero momentum transfer, we find scattering matrix elements in agreement with previous estimates for \si29 but significantly different than previous work for \ge73. A modest quenching, in accord with shell model studies of other heavy nuclei, has been included to bring agreement between the measured and calculated values of the magnetic moment for \ge73. Even with this quenching, the calculated scattering rate is roughly a factor of 2 higher than the best previous estimates; without quenching, the rate is a factor of 4 higher. This implies a higher sensitivity for germanium dark matter detectors. We also investigate the role of finite momentum transfer upon the scattering response for both nuclei and find that this can significantly change the expected rates. We close with a brief discussion of the effects of some of the non-nuclear uncertainties upon the matrix elements.